The aim of this study is to assess the performance of anaerobic digestion against co-digestion systems during the start-up stages based on key process parameters and biological indicators. Two parallel experiments treating sewage sludge alone or co-digested with low concentration of pig manure (8% vol., 2-3% in COD basis) were carried out in two lab-scale CSTR at mesophilic conditions. Same inoculant and organic loading rate sequences were applied for two consecutive runs of 79 and 90 days. According to the removal efficiencies achieved, no significant differences were encountered amongst mono-digestion and co-digestion. This observation was reinforced with the analysis of the total/active microbiome, sequencing 16S rRNA genes and transcripts. The addition of a co-substrate at low concentration had a negligible effect on the total/active microbial communities; they evolved following the same pattern. This might be an advantage in order to upgrade existing wastewater treatment plants to become centralized biogas facilities.